Have you tried changing the infill pattern to concentric? This will provide a better visual if it’s a plate vs filament flow issue. It will also confirm whether on not you have contamination on your plate.
Also have you tried chaning the pattern angle?
The sole reason for suggesting this is to rule out mechanical issues. The default infill is 45 Degrees. At 45 degrees you’re only moving one motor and belt.
Try this test to witness for yourself because it’s counterintuitive. Power off your machine, then move the head back and forth at a 45-degree angle. First from the 8 to 2 o’clock position, and then from 4 to 10 o’clock. You’ll notice that only one belt moves at a time when moving at 45 degrees. So, at 135 degrees, you’re isolating the other belt and motor, while at 45 degrees, the second motor is isolated.
If it is mechanical, then follow the maintenance steps on the wiki page for pulley lubrication and belt tensioning. Be very careful not to place lube anywhere near the belt.
By changing the pattern angle, you can control and isolate which motor/belt is moving. If it’s a mechanical issue with the belt or the motor, it should not be visible using the opposite direction of infill.
However, if after isolating the belt/motor, the problem stays in the same place on the plate, then no matter how clean you think your plate might be, it’s likely contaminated.
A less likely explanation but not impossible, is that your bed leveling is not happening correctly. Also, have you tried another plate just to rule out a defective plate?
One last question, can you hold up the test sample to a uniform light source so that we can see the transmissivity? I’m bothered by this perfect corner artifact where the lines seem to be making a perfect square(in green). That almost makes me want to tale a closer look at the slicer settings and/or the model itself. It would also help if you can show both a zoomed in and zoomed out screengrab of the sliced model to let us see the pattern being sliced.
